1. Field of the Invention
The present invention relates to a piezoelectric element used for piezoelectric ceramic resonators, piezoelectric ceramic filters, or the like.
2. Description of the Related Art
Conventionally, piezoelectric ceramic compositions containing lead zirconate titanate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a main component have widely been used for piezoelectric ceramic elements such as piezoelectric ceramic filters and piezoelectric resonators.
Since piezoelectric ceramic compositions containing lead zirconate titanate or lead titanate as a main component contain a large amount of lead, there is a problem in that the uniformity of the products deteriorate because of the vaporization of lead oxide in the manufacturing process.
In order to resolve the above problem, various piezoelectric ceramic compositions containing no lead or only a small amount of lead have been examined. Such piezoelectric ceramic compositions include materials containing a bismuth layer compound, such as strontium bismuth niobate (SrBi2Nb2O9), as a main-component. Since piezoelectric ceramic compositions containing a bismuth layer compound as a main component contain no lead oxide, the above problem does not arise.
In piezoelectric ceramic elements, such as piezoelectric ceramic filters and piezoelectric ceramic resonators, comprising piezoelectric ceramic compositions containing lead zirconate titanate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a main component, it has conventionally been known that piezoelectric elements having a layered structure including a plurality of piezoelectric layers and electrodes have high performance characteristics.
In piezoelectric ceramic elements comprising piezoelectric ceramic compositions containing lead zirconate titanate (Pb(TixZr1xe2x88x92x)O3) or lead titanate (PbTiO3) as a main component, platinum (Pt), having a melting point of 1,770xc2x0 C., and palladium (Pd), having a melting point of 1,550xc2x0 C., which are resistant to transpiration at high firing temperatures, e.g. about 1,100xc2x0 C., are usually used for the electrodes.
With respect to piezoelectric ceramic elements comprising piezoelectric ceramic compositions containing a bismuth layer compound as a main component, no examples of electrodes containing Pd as a main component were found by the inventors during their research.
Furthermore, the inventors have found that piezoelectric elements which have Pd electrodes and piezoelectric ceramic compositions containing a bismuth layer compound as a main component and which are prepared by firing cannot be made successfully. That is, the characteristics of the piezoelectric elements are not measurable, as shown in the following example.
It is reported in the Journal of the American Ceramics Society, vol. 75, p 2339 that a mixed powder of bismuth oxide and palladium oxide form a compound at a temperature higher than 500xc2x0 C., and the compound has a melting point of at least 800xc2x0 C.
The above report does not mention the reaction of a bismuth layer compound and metallic palladium but does mention the reaction of bismuth oxide and palladium oxide. However, it is not clear if the reason why piezoelectric elements which having Pd electrodes and piezoelectric ceramic compositions containing a bismuth layer compound as a main component and which are prepared by firing cannot be made successfully is the same as in the above report. There is a possibility that the firing of the untreated piezoelectric elements causes these unsatisfactory characteristics.
Accordingly, it is an object of the present invention to provide a piezoelectric element including Pd electrodes and a piezoelectric ceramic composition containing a bismuth layer compound as a main component in order to avoid the use of lead.
In order to solve the above problem, the piezoelectric element of the present invention includes a plurality of piezoelectric layers, each layer comprising a piezoelectric ceramic composition containing a bismuth layer compound and electrodes containing Pd. The piezoelectric layers are layered in the thickness direction of the piezoelectric element, the electrodes are provided to each of the piezoelectric layers, and at least one of the electrodes and the piezoelectric ceramic composition contain Ag.
In the above configuration, since Ag is contained in at least one of the piezoelectric ceramic composition and the electrodes, the diffusion of Pd contained in the electrodes into the piezoelectric ceramic composition and the reaction of Pd with the piezoelectric ceramic composition are inhibited, thus preventing the characteristics of the piezoelectric element from deteriorating when the piezoelectric ceramic composition containing a bismuth layer compound as a main component is used for the piezoelectric element.
In the above configuration, if m represents the weight of Ag contained in at least one of the electrodes and the piezoelectric ceramic composition and M represents the weight of Pd contained in the,electrodes, preferably the ratio of m to M is 0.1 m/M, and most preferably 0.1 n/M 10.
The piezoelectric ceramic composition preferably contains at least one compound selected from the group consisting of Bi2WO6, CaBi2Nb2O9, SrBi2Nb2O9, BaBi2Nb2O9, PbBi2Nb2O9, CaBi2Ta2O9, SrBi2Ta2O9, BaBi2Ta2O9, PbBi2Ta2O9, Bi3TiNbO9, Bi3TiTaO9, Bi4Ti3O12, SrBi3Ti2NbO12, BaBi3Ti2NbO12, PbBi3Ti2NbO12, CaBi4Ti4O15, SrBi4Ti4O15, BaBi4Ti4O15, PbBi4Ti4O15, Na0.5Bi4.5Ti4O15, K0.5Bi4.5Ti4O15, Ca2Bi4Ti5O18, Sr2Bi4Ti5O18, Ba2Bi4Ti5O18, Pb2Bi4Ti5O18, Bi6Ti3WO18, and the like, as a main component.
In the above configuration, since a bismuth layer compound is a main compound, a piezoelectric element such as a piezoelectric ceramic filter and a piezoelectric ceramic resonator, which contain no lead or a small amount of lead and are environmentally friendly, can be provided at a relatively low cost.
Japanese Unexamined Patent Application No. 12-007999 discloses that a piezoelectric ceramic composition containing a compound represented by formula SrBi2(Nb2xe2x88x92yWy)O9 can be adapted to materials for piezoelectric ceramic elements which are prepared by firing at a temperature lower than 1,100xc2x0 C. and have a electromechanical coefficient of a practical level. Such a piezoelectric ceramic composition may be used for the above piezoelectric layers.